Many output buffer circuits are coupled to one or more power supplies and use one or more output drivers to switch an output voltage according to the values of one or more inputs. For example, an output buffer that receives a high voltage (VDDHV) from a first power supply and a chip core voltage (VDD) from a second power supply might have p-channel and n-channel output drivers to switch an output voltage according to the values of a data input and an enable input. It is often desirable to protect the gate oxides of these output drivers and other components of the output buffer from overstress, break down, or other damage due to voltage levels on the output. Typically, the gate oxide of a transistor can withstand DC voltages only up to approximately VDD plus a transistor threshold voltage, a fact which motivates much of the circuitry of the present invention.
As microelectronic devices become increasingly complex to satisfy additional processing requirements, reducing the failure of devices during operation becomes increasingly important. A known technique for protecting the gate oxide of an n-channel output driver includes coupling a cascode device between the output driver and the output to shield the gate oxide of the output driver from voltage levels on the output. The gate of the cascode is maintained at a substantially constant intermediate voltage, such that the voltage across the gate oxide does not exceed the gate oxide's maximum fixed voltage tolerance. Although these techniques protect the output driver gate oxide when the cascode is functioning properly, the output driver is susceptible to failure if the cascode device has shorted, even though the output buffer may appear to be functioning properly. Furthermore, such techniques do not provide for detection of cascode failure, leaving previous output buffers and the associated electronic devices prone to unexpected failure during operation. These and other inadequacies make prior output buffers unsuitable for many applications.